The present invention relates to a data reproduction apparatus and method for performing reproduction of data from an optical disk.
FIG. 4 is a schematic view showing a common data recording/reproducing system for an optical disk. In the recording and reproduction of data on an optical disk, there are two types of velocity control manners (methods): a constant linear velocity (CLV) method and a constant angular velocity (CAV) method.
The CLV method is referred to as a method which controls a spindle motor according to the position of a disk so that the linear velocity of a data track which is traced by the head is made constant at all times. The CLV method has the advantage that data can be recorded with constant recording density over the entire disk, but on the other hand, since the rotational speed of the disk has to be changed according to the head position, it has the disadvantages that circuitry is increased in size for the speed change control and time is required for searching.
The CAV method is referred to as a method in which the number of revolutions of a spindle motor is always constant. For this reason, the CAV method has the advantages that circuitry can be reduced in size and also searching can be easily performed, because changing the speed of the spindle motor is unnecessary compared with the CLV method. However, the CAV method has the disadvantage that the total amount of recorded data is reduced, because if data is written to both an inner track and an outer track by the same clock signal, a difference in recording density will occur.
Now, a description will be made of a specific reproduction example of data. Each track on an optical disk is usually constituted by a main data area for main data, such as video data, voice data, and system data, and a secondary data area for secondary data such as a track address representing an address of a track. The main data area occupies the greater part of an area on the disk, and even if data of any frequency were reproduced, the data and the clock signal would be extracted through a phase-locked loop (PLL) circuit and therefore the reproduction of the data could be performed.
On the other hand, the secondary data such as a track address is not long enough to be passed through a PLL circuit, because it is constituted by the minimum number of data bits for recording main data as much as possible. However, the track address is a key representing the information of the track, and in the case where the track address is not reproduced correctly, there are cases where the incorrectly reproduced track address will damage the data reproduction processing thereafter. However, in the case where recording and reproduction are performed between the same methods, data of a recorded frequency is reproduced, so a data reproduction unit can be relatively easily constructed.
In FIG. 5 there is shown a specific format of the track address (TA). In the TA here, the track is divided into three blocks, each containing a synchronous idle character (SYNC) area, a track address (TA) area, and a cyclic redundancy check character (CRCC) area, and a preamble (PR) area and a postamble (PO) area are provided at the start and the end of the track, respectively.
Next, in FIG. 6 a recording method and a clock phase relationship are shown. In the recording method, assume that frequency modulation (FM or F2F) recording by a double clock signal is performed to convert a "0" data bit to 10 data bits and a "1" data bit to 11 data bits and, furthermore, non-return-to-zero (NRZ)-inverted (NRZI) recording is performed. At this time, if it is assumed that a write clock signal with respect to a "1" data bit in the track address (TA) data bits is a track address clock (TACK) signal, the recording cycle of data bits will be a double track address clock (2TACK) interval which is double the TACK interval by FM recording. Furthermore, since NRZI recording is performed, a "0" data bit is inverted at the track address clock (TACK) interval and a "1" data bit is continuously inverted twice at the double track address clock (2TACK) interval. Moreover, if the PR and PO data bits are all made 0's, then they will be repetitive data bits with a track address clock (TACK) interval. For secondary data such as a track address TA, recording and reproduction are performed not by a fast clock signal but by a relatively slower clock signal since no PLL is applied and some jitter occurs in reproduction.
In an actual detection method, data is sampled at double track address clock (2TACK) intervals and followed by the track address TA. If the synchronous idle character (SYNC) with a unique waveform which is not usually reproduced from other signals is detected, then the track address (TA) and the cyclic redundancy check character (CRCC) will be reproduced. With the CRCC, it is checked whether or not the SYNC contains an error. The waveform of SYNC is shown in FIG. 8. This waveform is obtained after FM recording and NRZI recording and is a relatively long unique pattern that is not detected from the PR, TA, or CRCC.
FIG. 11 shows how the track address (TA) is reproduced by a conventional method. Since reproduced data contains slight jitter between data bits, data is sampled at the rising and falling edges of the pulse of the double track address clock (2TACK) signal whose cycle is greater than an amount of jitter, as shown at 72 and 73. With this, if either cyclic redundancy check character (CRCC) contains no error by performing SYNC detection operations at 74 and 75, TA data detection operations at 76 and 77, and CRCC checking operations at 78 and 79, then it can be confirmed that the track address (TA) has been reproduced correctly.
As described above, in the recording and reproduction of data on an optical disk, there are two types of methods: a CLV method and a CAV method, and each method has advantages and disadvantages. For recording, the CLV method is superior to the CAV method in that data can be written with constant recording density over the entire disk surface. For reproduction, in the case where a recording/reproduction unit consists of a single head and produces a single output signal, the same CLV method may be employed. However, in the case where a recording/ reproduction unit consists of a plurality of heads, when a multitude of output signals are made possible or an editing operation requiring the switching of a plurality of systems is performed, it is realized only by the CAV method in which the speed of a spindle motor is always constant and also searching is easy. The reason for this is that since the CLV method performs control of rotation in accordance with the position of a track at which a recording/reproduction head is positioned, the CLV method cannot make a plurality of heads active at the same time.
In order to utilize the respective advantages of the two methods, it is necessary that recording and reproduction can be performed between different methods. For example, it is necessary that the data on an optical disk recorded with the CLV method can be reproduced with the CAV method. In such a case, the frequency of recorded data will differ from that of reproduced data.
As previously described, in the case where recording and reproduction are performed with the same method, data of a frequency recorded is reproduced, and consequently, the data reproduction unit can be constructed relatively easily. However, in the case where data is recorded and reproduced with different methods, reproduced data differs in frequency from recorded data, and the data reproduction unit cannot be easily constructed. More specifically, in the case where data recorded with the CLV method is reproduced with the CAV method, the frequency of data to be reproduced will be higher than that of recorded data if the relative speed between the reproduction head and the optical disk is faster than that used during recording, and a lower reproduced frequency will be obtained if the relative speed is slower. Thus, the frequency varies between recording and reproduction, and furthermore, it is not easy to reproduce a frequency which varies gradually depending upon the position of the reproduction head.
While the aforementioned recording has been performed with a clock-locked signal, there is another method which records data with a variable clock signal so that a reproduced frequency becomes constant when reproduced. Thus, there are various methods for recording and reproduction, and circuitry which can record and reproduce data only by a predetermined method has hitherto been constructed.